I operated the PA31 as a co-pilot for about 300hrs last year. This role included either acting as the pilot flying, or when acting as the pilot not flying; all operations of the flight except actually having my hands on the controls.
The POH specifies the aircraft can be operated two crew, our company ops manual requires a co-pilot, CAR's say I can operate as a co-pilot, CAO48 etc requires me to log it as flight-time (though my company prohibits me from doing this).
Story from the Darwin FOI's is that it can't be logged, even though as a crew member, I'm still legally responsible for the conduct of the flight.
Frustrating, as I'd have another 300hrs multi otherwise!